1. Field of the Invention
The present invention relates to fine coated copper particles of a nanometer size that have superior dispersibility in a solvent and demonstrate favorable electrical conductivity as a result of low-temperature sintering on a flexible printed board and the like, a method for producing the same, and a sintered copper adherend using the same.
2. Description of Related Art
In the background of recent remarkable progress of electronic devices, there exist advances of semiconductor devices and other electronic components, and the considerable developments of printed wiring substrates on which these electronic components are mounted. For many of these electronic devices there is a need for compact size, reduced thickness, and lightweight, and furthermore, improved productivity. Therefore, printed wiring substrates have also been required to have various contrivances and improvements to deal with this need. In particular, to achieve these requirements, there is a demand for higher speed and higher density to mount the material of electrically conductive wiring for the electronic components.
Amidst such circumstances, nanosized fine metal particles are currently expected to serve as one of such materials, and studies for this are now being conducted (for example, see Kawazome, M. et al., Pulverization, No. 50, 27-31 (2006/2007)).
As the fine metal particles to be used as such a wiring material, ones mainly formed of copper are preferable from the point of low electromigration. For this reason, means for providing nanosized fine copper particles have so far been studied. For example, in Japanese Unexamined Patent Publication No. 2010-24526, it is described that, as a method for producing fine copper particles, a fine copper powder is produced by reducing a precursor of fine copper particles by a treatment with a hydrazine derivative in water. However, forming a copper coating by sintering the thus produced fine copper powder at low temperatures has yet to be achieved. In addition, Japanese Unexamined Patent Publication No. 2007-321215 discloses a technique, as a method for producing fine copper particles, in which metal nanoparticles are produced by reducing an organic acid salt of a metal by treating a solution containing the metal salt and an amine with a reducing agent. It is described that the fine copper particles produced by this method attained electrical conductivity by sintering at about 300° C.
In the methods for producing fine copper particles according to the methods described in Japanese Unexamined Patent Publication No. 2010-24526 and Japanese Unexamined Patent Publication No. 2007-321215, metal copper is generated by previously preparing a copper-containing compound in a liquid, and charging a reducing agent thereto from the outside to cause the reduction reaction. Thus, it is inevitable for the speed of the progression of the reduction reaction to be limited by the supply of the reducing agent and such substances. For this reason, in a case of an actual production of fine copper particles, it is difficult to cause a reaction to generate uniform fine copper particles, due to uneven concentrations of the raw material substances in the reaction container, and the like. Thus, in particular, it becomes very difficult to uniformly produce a large amount of fine copper particles in the industrial production process, which is a problem.
On the other hand, Japanese Unexamined Patent Publication No. 2008-214695 discloses a technique for producing fine silver particles protected by protective films of oleylamine by: mixing silver oxalate with an excessive amount of oleylamine to allow them to react so as to form a complex compound; and thereafter heating the complex compound to decompose oxalate ions included in the complex compound so as to generate atomic silver. It is described that the method is able to provide fine silver particles which are ultra-fine with a narrow particle size distribution and superior storage stability.
According to this method, silver oxalate is thermally decomposed in oleylamine to generate atomic silver, and the aggregation thereof enables the production of fine silver particles. In this production method, silver atoms are generated by the decomposition of a single type of molecule, because of which, atomic silver can be generated evenly in the reaction system without a limitation on the speed of the reaction due to the supply of substances. Moreover, it is possible, by controlling the aggregation of atomic silver with the action of oleylamine, to produce fine silver particles with clean surfaces which are ultra-fine with a narrow particle size distribution and superior storage stability. As a result, the use of the fine silver particles produced by this method makes it possible to provide an ink for wiring formation with which wiring can be formed by sintering on the surface of a resin.
Regarding copper-containing compounds, if it is possible to generate atomic copper by self decomposition in a predetermined alkylamine, as described in Japanese Unexamined Patent Publication No. 2008-214695, it is also expected to produce fine copper particles coated with an alkylamine similarly to the fine silver particles.
However, generally speaking, a copper-containing compound has a large free energy change in its formation. Thus, mere heating is not always enough to generate atomic copper by self decomposition of the compound. Also, if a mixture containing the compound and an alkylamine is strongly heated; the alkylamine may be vaporized or degraded before the generation of atomic copper, another chemical reaction differing from the copper reduction reaction may occur between the compound and the alkylamine, or other such problems may occur. Thus, it is difficult to produce superior fine copper particles. For this reason, what is applicable to the method described in Japanese Unexamined Patent Publication No. 2008-214695 has been limited to compounds which are mainly composed of silver.